In general, electrolytic capacitors are prepared by immersing with an electrolyte and electrolytic capacitors element comprising separating paper located between electrode foils at a cathode and an anode, and then sealing in an armoring case. As is well known, an aluminum foil or the like which has undergone an etching treatment is employed as the electrode foils; as the electrode foil at the anode, the etching-treated foil followed by formation of an oxidized layer of dielectrics is employed. In such electrolytic capacitors, an electrolyte in which electrolytic capacitor elements are immersed is a main constituent of electrolytic capacitors, together with electrode foils, separating paper, armoring cases and sealing members, etc. The chemical or electric properties of an electrolyte become factors which determine the electric properties and life properties of the electrolytic capacitors.
Electrolytes used for medium and high voltage electrolytic capacitors having a rated voltage of more than 160 V are a so called ethylene glycol-boric acid type electrolyte containing boric acid or an ammonium salt thereof in a solvent mainly composed of ethylene glycol (refer to U.S. Pat. No. 3,812,039). It is known that the electrolyte of this type increases an equivalent series resistance of an electrolytic capacitor and at the same time, increases its loss, due to markedly high specific resistance as compared to that of an electrolyte for low voltage electrolytic capacitors. Further, the electrolyte of this type also contains large amounts of water formed during esterification of ethylene glycol and boric acid, in addition to water inherently contained in chemicals constituting the electrolyte. These waters seriously deteriorate an oxidized layer of a dielectric formed at the surface of electrode foil at the anode so that electric properties of the electrolytic capacitor are rendered unstable. This results in reduction of the life of the capacitor. In addition, electrolytic capacitors are accompanied by the generation of heat once the ripple current is turned on, according to the amount of the ripple current and the internal resistance. Depending upon condition for use, the temperatures of electrolytic capacitors can become higher than 100.degree. C. The water contained in electrolyte forms large amounts of water vapor at temperatures exceeding 100.degree. C. which abnormally increases the inner pressure of an armoring case. This can cause distortion in the appearance of the armoring case and deteriorate the electric properties. For these reasons, it is impossible to use the electrolyte of this type at temperatures over 105.degree. C. Accordingly, the upper limit for temperatures at which conventional electrolytic capacitors are employed is restricted.
Electrolytic capacitors are used in a variety of different types of electronic equipment, such as communication equipment and measuring equipment. As is well known, electric properties of electrolytic capacitors play a large part in the efficiencies of these electronic equipments. In order to render these electronic equipments compact and highly efficient, electrolytic capacitors having long life, excellent electric properties and high reliance have been desired. Particularly in stable electric sources, switching regulators having a high switching frequency are employed based on requirements such rendering equipments compact. Smoothing capacitors employed for an electric source of this type must have low impedance characteristics to a high frequency and at the same time, have a high upper temperature-limitation.
To comply with such a requirement, an electrolyte using a long chain dibasic acid having an alkyl group at the side chain thereof has been proposed in Japanese patent application (OPI) No. 27013/82. Such an electrolyte is used to reduce the specific resistivity affecting impedance characteristics of electrolytic capacitors and broaden the temperature range, particularly to increase the upper temperature limit under which the capacitor can be used. That is, the electrolyte obtained by dissolving a long chain dibasic acid having an alkyl group substituent in a solvent mainly composed of ethylene glycol exhibits an extremely small specific resistance as compared to ethylene glycol-boric acid type electrolytes and can markedly reduce an equivalent series resistance and its loss of the electrolytic capacitor. Further, in the case of conventional ethylene glycol-boric acid type electrolytes, esterification easily proceeds to release 3 mols of water which is obtained by a condensation reaction from 1 mol of boric acid. Accordingly, the water content in the electrolytes becomes extremely large. However, with electrolytes containing long chain dibasic acids therein, the water content becomes extremely small since the amount of a solute is smaller than that of conventional electrolytes and the water obtained by condensation reaction is produced in an extremely small amount due to the larger molecular weight of the long chain dibasic acids as compared to the molecular weight of boric acid. For this reason, deterioration of an oxidized layer of a dielectric which is formed at the surface of the electrode foil at anode can be prevented.
However, with electrolytes of this kind, there is a tendency when using an etched aluminum foil as a cathode that electric capacity might be decreased due to high facility of forming a complex of the long chain dibasic acid to aluminum. That is an aluminum foil which has undergone an etching treatment is employed as the electrode foil at the cathode. When the long chain dibasic acid is brought into contact with this aluminum foil, a complex is formed at the surface thereof. The etching treatment to an electrode foil leads to forming fine etching pits at the surface and interior of the electrode foil to enlarge an effective surface area of the electrode foil. The complex thus formed is formed such that it shortens the etching pits and acts to reduce the effective surface area of the electrode foil at the cathode. It is thus believed that the decrease in electric capacity would be due to such reduction of the effective surface area. In addition, such a complex is formed in the presence of a slight amount of water, for example, water brought during the assembly line of products. Therefore, the electric capacity of the electrolytic capacitor is reduced even during life test.